Reproducing objective for video disks

ABSTRACT

A reproducing objective for video disks comprising five single lenses for which the working distance is large and flatness of image and resolving power are high.

BACKGROUND OF THE INVENTION

1. Field of the invention

The present invention relates to a reproducing objective for video disksand, more particularly, to a reproducing objective for reading out thesignals recorded on high-density information-recording disks (videodisks).

2. Description of the prior art

For objectives to be used in reproducing systems for video disks, it isrequired to warrant resolving power of 1 μ due to the fact that theobjective has to read out very small signals recorded with high density.Moreover, the information read out from the disk, which rotates at highspeed, contains information for making the objective follow up therecorded track and information for automatic focusing in addition toimage information. To make the objective read out those kinds ofinformation correctly, the flatness of image focused by the objectiveshould be high. To prevent breakage of the video disk and objectivewhich will be caused when the objective contacts the video disk, theworking distance of the objective should be long. Besides, to performautomatic focusing, the objective should be compact and light in weight.Moreover, the price of the objective should be low. As the light usedfor the objective for video disks is generally a monochromatic light ([.= 632.8 mm), it is effective for eliminating the noise at the time ofamplifying the signals from a detector when transparency for the lightof this wavelength is as high as possible. Therefore, to maketransparency high, it is necessary to provide multi-layeranti-reflection coating on the lens surfaces or to make the number oflenses constituting the objective as small as possible. When thisproblem is considered in connection with the above-mentioned otherrequirements such as low price and light weight, it is more advantageouswhen the number of lenses constituting the objective is made as small aspossible.

SUMMARY OF THE INVENTION

It is, therefore, a primary object of the present invention, to providea reproducing objective for video disks for which the working distanceis large, flatness of image is high and resolving power is high. For theobjective according to the present invention, the Gauss type lensconfiguration is adopted as it is easier to obtain a large workingdistance and high flatness of image by the Gauss type lensconfiguration. As shown in FIG. 1, the objective for video disksaccording to the present invention is a modified Gauss type lens systemcomprising five single lenses, i.e., a first, second, third, fourth andfifth lenses. The first lens is a positive lens with its planar orconcave surface positioned toward the object side, the second lens is apositive lens, the third lens is a negative meniscus lens with itsconvex surface positioned toward the object side, the fourth lens is anegative meniscus lens with its concave surface positioned toward theobject side, and the fifth lens is a positive meniscus lens with itsconcave surface positioned toward the object side. Besides, theobjective for video disks according to the present invention satisfiesthe following conditions when reference symbol f represents the focallength of the lens system as a whole, reference symbol f₃ represents thefocal length of the third lens, reference symbol f₄ represents the focallength of the fourth lens, reference symbol r₆ represents the radius ofcurvature of the surface on the image side of the third lens, referencesymbol r₇ represents the radius of curvature of the surface on theobject side of the fourth lens, reference symbol d₁ represents thethickness of the first lens, and reference symbol d₅ represents thethickness of the third lens.

    1. 0.30 ≧ r6/f ≧ 0.23

    2. 0.38 ≧ dl/f ≧ 0.26

    3. 1.4 ≧ r6/|r7| ≧ 0.9

    4. 6.0 ≧ |f3|/|f4| ≧ 2.5

    5. 0.31 ≧ d5/f ≧ 0.21

When, in the lens system of the above-mentioned configuration, r₆ /fbecomes larger than the upper limit of the condition (1), i.e., r₆/f>0.3, distortion will increase and spherical aberration in the zonaland marginal portions will be overcorrected. When, on the contrary, r₆/f becomes smaller than the lower limit, i.e., r₆ /f<0.23, astigmatismwill be undercorrected. When d₁ /f in the condition (2) becomes d₁/f>0.38, the working distance becomes short and it becomes impossible toattain the object of the present invention. Moreover, sphericalaberration will be undercorrected and astigmatic difference becomeslarge. When, on the contrary, it becomes d₁ /f<0.26 coma and sphericalaberration in the marginal portion will become unfavourable though theworking distance becomes large.

If it becomes r₆ /| r₇ |>1.4 in the condition (3), spherical aberrationand coma will be overcorrected. If it becomes r₆ / r₇ >0.9, coma will besomewhat undercorrected. If it becomes |f₃ |/|f₄ |<2.5 in the condition(4), coma and spherical aberration in the marginal portion will becomelarge. If it becomes |f₃ |/|f₄ |>6.0, symmetry of coma will becomeunfavourable. Moreover, astigmatism will be undercorrected and willincrease in negative direction toward the marginal portion. If, in thecondition (5), it becomes d₅ /f>0.31, spherical aberration will besomewhat overcorrected and, at the same time, astigmatic differencebecomes large. If, on the contrary, it becomes d₅ /f<0.21, coma will besomewhat overcorrected. As described in the above, by the objectiveaccording to the present invention, it is possible to attain the objectof the present invention. For the objective according to the presentinvention, it is more preferable to use materials of high refractiveindices for the third and fourth lenses due to the following reason.That is, in order to favourably correct curvature of field, coma, etc.of the lens system, it is unavoidable to arrange so that radii ofcurvature r₆ and r₇ of the surface on the image side of the third lensand surface on the object side of the fourth lens respectively becomesmall. When, however, refractive indices of these lenses are high, it ispossible to make radii of curvatures r₆ and r₇ larger compared with thecase that these refractive indices are low. As a result, it becomeseasier to manufacture these lenses and the cost of production is reducedaccordingly. Therefore, for refractive indices n₃ and n₄ of the thirdand fourth lenses, it is more preferable to make them n₃ ≧ 1.6 and n₄ ≧1.6.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sectional view of Embodiments 1 through 3 of theobjective according to the present invention;

FIGS. 2A, 2B, 2C, 2D and 2E respectively show graphs illustratingaberration curves of Embodiment 1;

FIGS. 3A, 3B, 3C, 3D and 3E respectively show graphs illustratingaberration curves of Embodiment 2;

FIGS. 4A, 4B, 4C, 4D and 4E respectively show graphs illustratingaberration curves of Embodiment 3;

FIG. 5 shows a sectional view of Embodiments 4 and 5;

FIGS. 6A, 6B, 6C, 6D and 6E respectively show graphs illustratingaberration curves of Embodiment 4; and

FIGS. 7A, 7B, 7C, 7D and 7E respectively show graphs illustratingaberration curves of Embodiment 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the reproducing objective for video disksaccording to the present invention are as shown below.

Embodiment 1

r₁ = ∞

d₁ = 0.2825 n₁ = 1.51463 ν₁ = 64.2

r₂ = -0.5927

d₂ = 0.025

r₃ = 1.1536

d₃ = 0.1969 n₂ = 1.51463 ν₂ = 64.2

r₄ = -0.895

d₄ = 0.1154

r₅ = 0.4044

d₅ = 0.2789 n₃ = 1.69426 ν₃ = 30.1

r₆ = 0.2729

d₆ = 0.2681

r₇ = -0.2129

d₇ = 0.3165 n₄ = 1.69426 ν₄ = 30.1

r₈ = -0.4235

d₈ = 0.0458

r₉ = -44.5212

d₉ = 0.2232 n₅ = 1.61656 ν₅ = 36.3

r₁₀ = -0.987

f = 1, f₃ = -9.252, f₄ = -1.606 ΣP = 0.015, l = 0.175

Embodiment 2

r₁ = ∞

d₁ = 0.2825 n₁ = 1.51463 ν₁ = 64.2

r₂ = -0.4723

d₂ = 0.025

r₃ = 0.8966

d₃ = 0.1969 n₂ = 1.51463 ν₂ = 64.2

r₄ = -1.4074

d₄ = 0.1154 r₅ = 0.4002

d₅ = 0.2789 n₃ = 1.69426 ν₃ = 30.1

r₆ = 0.2623

d₆ = 0.2681

r₇ = -0.2085

d₇ = 0.3079 n₄ = 1.69426 ν₄ = 30.1

r₈ = -0.4192

d₈ = 0.0833

r₉ = -7.3497

d₉ = 0.2232 n₅ = 1.61656 ν₅ = 36.3

r₁₀ = -0.8788

f = 1, f₃ = -5.131, f₄ = -1.490 ΣP = 0.015, l = 0.175

Embodiment 3

r₁ = ∞

d₁ = 0.2825 n₁ = 1.51463 ν₁ = 64.2

r₂ = -0.5232

d₂ = 0.025

r₃ = 0.9794

d₃ = 0.1969 n₂ = 1.51463 ν₂ = 64.2

r₄ = -1.0976

d₄ = 0.1154

r₅ = 0.4027

d₅ = 0.2789 n₃ = 1.69426 ν₃ = 30.1

r₆ = 0.2656

d₆ = 0.2681

r₇ = -0.2095

d₇ = 0.3079 n₄ = 1.69426 ν₄ = 30.1

r₈ = -0.4211

d₈ = 0.0833

r₉ = -11.1307

d₉ = 0.2232 n₅ = 1.61656 ν₅ = 36.3

r₁₀ = -0.9144

f = 1, f₃ = -6.746, f₄ = -1.486 ΣP = 0.015, l = 0.175

Embodiment 4

r₁ = -2.0856

d₁ = 0.366 n₁ = 1.72309 ν₁ = 28.5

r₂ = -0.6253

d₂ = 0.0623

r₃ = 0.7613

d₃ = 0.2258 n₂ = 1.60018 ν₂ = 38.0

r₄ = 13.1246

d₄ =0.0672

r₅ = 0.406

d₅ = 0.2349 n₃ = 1.60018 ν₃ = 38.0

r₆ = 0.2756

d₆ = 0.3411

r₇ = -0.2656

d₇ = 0.3154 n₄ = 1.72309 ν₄ = 28.5

r₈ = -0.538

d₈ =0.0141

r₉ = -7.5535

d₉ = 0.1627 n₅ = 1.60018 ν₅ = 38.0

r₁₀ = -0.8135

f = 1, f₃ = -4.415, f₄ = -1.534 ΣP = 0.015, l = 0.21

Embodiment 5

r₁ = -2.8161

d₁ = 0.367 n₁ = 1.72309 ν₁ = 28.5

r₂ = -0.6254

d₂ = 0.602

r₃ = 0.7658

d₃ = 0.227 n₂ = 1.60018 ν₂ = 38.0

r₄ = 11.4344

d₄ = 0.0563

r₅ = 0.4057

d₅ = 0.2347 n₃ = 1.60018 ν₃ = 38.0

r₆ = 0.2763

d₆ = 0.3424

r₇ = -0.2666

d₇ = 0.3164 n₄ = 1.72309 ν₄ = 28.5

r₈ = -0.5258

d₈ = 0.0132

r₉ = -7.5383

d₉ = 0.1625 n₅ = 1.60018 ν₅ = 38.0

r₁₀ = -0.8161

f = 1, f₃ = -4.516, f₄ = -1.539 ΣP = 0.015, l = 0.212

In the above-mentioned respective embodiments, reference symbols r₁through r₁₀ respectively represent radii of curvature of respective lenssurfaces, reference symbols d₁ through d₉ respectively representthicknesses of respective lenses and airspaces between respectivelenses, reference symbols n₁ through n₅ respectively representrefractive indices of respective lenses, reference symbols ν₁ through ν₅respectively represent Abbe's numbers of respective lenses, referencesymbol f represents the focal length of the lens system as a whole,reference symbol f₃ represents the focal length of the third lens,reference symbol f₄ represents the focal length of the fourth lens,reference symbol l represents the working distance of the lens systemand reference symbol ΣP represents Petzval's sum. Embodiments 1 through3 have lens configuration as shown in FIG. 1. For these embodiments, thefirst lens surface r₁ is a planar surface. Embodiments 4 and 5 have lensconfiguration as shown in FIG. 5. For these embodiments, the first lenssurface r.sub. 1 is a concave surface. As explained in the above, thelens system according to the present invention fully satisfies therequirements for a reproducing objective for video disks. This is alsoevident from favourable corrected aberrations shown in respective graphsof aberrations curves. As for astigmatism of respective embodiments,each graph looks as if it shows only one curve. This is becauseastigmatic difference is very small and, consequently, the curve ofsagittal astigmatism almost coincides with the curve of meridionalastigmatism. When astigmatic difference of a lens system is not verysmall, it is impossible to make the image flat over a wide field. If, insuch case, true roundness of the video disk is low or tracks on the diskare eccentric, signals in different track will be read out because thesize of tracks on the disk is in the order of micron and, consequently,an error will be caused. Therefore, in the present invention, astigmaticdifference is made extremely small as shown in graphs of aberrationcurves.

We claim:
 1. A reproducing objective for video disks comprising a first,second, third, fourth and fifth lenses, said first lens being a positivelens, said second lens being a positive lens, said third lens being anegative meniscus lens with its convex surface positioned toward theobject side, said fourth lens being a negative meniscus lens with itsconcave surface positioned toward the object side, said fifth lens beinga positive meniscus lens with its concave surface positioned toward theobject side, said reproducing objective for video disks satisfying thefollowing conditions:

   
 1. 0.30 .increment. r.sub.6 /f ≧ 0.23

   
 2. 0.38 ≧ d.sub.1 /f ≧ 0.26

   
 3. 1.4 ≧ r.sub.6 /|r.sub.7 | ≧ 0.9

   
 4. 6.0 ≧ |f.sub.3 |/|f.sub.4  | ≧ 2.5

   
 5. 0.31 ≧ d.sub.5 /f ≧ 0.21

wherein referemce symbol f represents the focal length of the lenssystem as a whole, reference symbol f₃ represents the focal length ofthe third lens, reference symbol f₄ represents the focal length of thefourth lens, reference symbol r₆ represents the radius of curvature ofthe surface on the image side of the third lens, reference symbol r₇represents the radius of curvature of the surface on the object side ofthe fourth lens, reference symbol d₁ represents the thickness of thefirst lens and reference symbol d₅ represents the thickness of the thirdlens.
 2. A reproducing objective for video disks according to claim 1,in which said reproducing objective for video disks has the followingnumerical data:r₁ = ∞d₁ = 0.2825 n₁ = 1.51463 ν₁ = 64.2 r₂ = -0.5927d₂ =0.025 r₃ = 1.1536d₃ = 0.1969 n₂ = 1.51463 ν₂ = 64.2 r₄ = -0.895d₄ =0.1154 r₅ = 0.4044d₅ = 0.2789 n₃ = 1.69426 ν₃ = 30.1 r₆ = 0.2729d₆ =0.2681 r₇ = -0.2129d₇ = 0.3165 n₄ = 1.69426 ν₄ = 30.1 r₈ = -0.4235d₈ =0.0458 r₉ = -44.5212d₉ = 0.2232 n₅ = 1.61656 ν₅ = 36.3 r₁₀ = -0.987f =1, f₃ = -9.252, f₄ = -1.606 ΣP = 0.015, l = 0.175Wherein referencesymbols r₁ through r₁₀ respectively represent radii of curvature ofrespective lens surfaces, reference symbols d₁ through d₉ respectivelyrepresent thicknesses of respective lenses and airspaces betweenrespective lenses, reference symbols n₁ through n₅ respectivelyrepresent refractive indices of respective lenses, reference symbols ν₁through ν₅ respectively represent Abbe's numbers of respective lenses,reference symbol f represents the focal length of the lens system as awhole, reference symbol f₃ represents the focal length of the thirdlens, reference symbol f₄ represents the focal length of the fourthlens, reference symbol l represents the working distance of the lenssystem and reference symbol ΣP represents Petzval's sum.
 3. Areproducing objective for video disks according to claim 1, in whichsaid reproducing objective for video disks has the following numericaldata:r₁ = ∞d₁ = 0.2825 n₁ = 1.51463 ν₁ = 64.2 r₂ = -0.4723d₂ = 0.025 r₃= 0.8966d₃ = 0.1969 n₂ = 1.51463 ν₂ = 64.2 r₄ = -1.4074d₄ = 0.1154 r₅ =0.4002d₅ = 0.2789 n₃ = 1.69426 ν₃ = 30.1 r₆ = 0.2623d₆ = 0.2681 r₇ =-0.2085d₇ = 0.3079 n₄ = 1.69426 ν₄ = 30.1 r₈ = -0.4192d₈ = 0.0833 r₉ =-7.3497d₉ = 0.2232 n₅ = 1.61656 ν₅ = 36.3 r₁₀ = -0.8788f = 1, f₃ =-5.131, f₄ = -1.490 ΣP = 0.015, l = 0.175Wherein reference symbols r₁through r₁₀ respectively represent radii of curvature of respective lenssurfaces, reference symbols d₁ through d₉ respectively representthicknesses of respective lenses and airspaces between respectivelenses, reference symbols n₁ through n₅ respectively representrefractive indices of respective lenses, reference symbols ν₁ through ν₅respectively represent Abbe's numbers of respective lenses, referencesymbol f represents the focal length of the lens system as a whole,reference symbol f₃ represents the focal length of the third lens,reference symbol f₄ represents the focal length of the fourth lens,reference symbol l represents the working distance of the lens systemand reference symbol ΣP represents Petzval's sum.
 4. A reproducingobjective for video disks according to claim 1, in which saidreproducing objective for video disks has the following numericaldata:r₁ = ∞d₁ = 0.2825 n₁ = 1.51463 ν₁ = 64.2 r₂ = -0.5232d₂ = 0.025 r₃= 0.9794d₃ = 0.1969 n₂ = 1.51463 ν₂ = 64.2 r₄ = -1.0976d₄ = 0.1154 r₅ =0.4027d₅ = 0.2789 n₃ = 1.69426 ν₃ = 30.1 r₆ = 0.2656d₆ = 0.2681 r₇ =-0.2095d₇ = 0.3079 n₄ = 1.69426 ν₄ = 30.1 r₈ = -0.4211d₈ = 0.0833 r₉ =-11.1307d₉ = 0.2232 n₅ = 1.61656 ν₅ = 36.3 r₁₀ = -0.9144f = 1, f₃ =-6.746, f₄ = -1.486 ΣP = 0.015, l = 0.175Wherein reference symbols r₁through r₁₀ respectively represent radii of curvature of respective lenssurface, reference symbols d₁ through d₉ respectively representthicknesses of respective lenses and airspaces between respectivelenses, reference symbols n₁ through n₅ respectively representrefractive indices of respective lenses, reference symbols ν₁ through ν₅respectively represent Abbe's numbers of respective lenses, referencesymbol f represents the focal length of the lens system as a whole,reference symbol f₃ represents the focal length of the third lens,reference symbol f₄ represents the focal length of the fourth lens,reference symbol l represents the working distance of the lens systemand reference symbol ΣP represents Patzval's sum.
 5. A reproducingobjective for video disks according to claim 1, in which saidreproducing objective for video disks has the following numericaldata:r₁ = -2.0856d₁ = 0.366 n₁ = 1.72309 ν₁ = 28.5 r₂ = -0.6253d₂ =0.0623 r₃ = 0.7613d₃ = 0.2258 n₂ = 1.60018 ν₂ = 38.0 r₄ = 13.1246d₄ =0.0672 r₅ = 0.406d₅ = 0.2349 n₃ = 1.60018 ν₃ = 38.0 r₆ = 0.2756d₆ =0.3411 r₇ = -0.2656d₇ = 0.3154 n₄ = 1.72309 ν₄ = 28.5 r₈ = -0.538d₈ =0.0141 r₉ = -7.5535d₉ = 0.1627 n₅ = 1.60018 ν₅ = 38.0 r₁₀ = -0.8135f =1, f₃ = -4.415, f₄ = -1.534 ΣP = 0.015, l = 0.21Wherein referencesymbols r₁ through r₁₀ respectively represent radii of curvature ofrespective lens surface, reference symbols d₁ through d₉ respectivelyrepresent thicknesses of respective lenses and air spaces betweenrespective lenses, reference symbols n₁ through n₅ respectivelyrepresent refrative indices of respective lenses, reference symbols ν₁through ν₅ respectively represent Abbe's numbers of respective lenses,reference symbol f represents the focal length of the lens system as awhole, reference symbol f₃ represents the focal length of the thirdlens, reference symbol f₄ represents the focal length of the fourthlens, reference symbol l represents the working distance of the lenssystem and reference symbol ΣP represents Petzval's sum.
 6. Areproducing objective for video disks according to claim 1, in whichsaid reproducing objective for video disks has the following numericaldata:r₁ = -2.8161d₁ = 0.367 n₁ = 1.72309 ν₁ = 28.5 r₂ = -0.6254d₂ =0.602 r₃ = 0.7658d₃ = 0.227 n₂ = 1.60018 ν₂ = 38.0 r₄ = 11.4344d₄ =0.0563 r₅ = 0.4057d₅ = 0.2347 n₃ = 1.60018 ν₃ = 38.0 r₆ = 0.2763d₆ =0.3424 r₇ = -0.2666d₇ = 0.3164 n₄ = 1.72309 ν₄ = 28.5 r₈ = -0.5258d₈ =0.0132 r₉ = -7.5383d₉ = 0.1625 n₅ = 1.60018 ν₅ = 38.0 r₁₀ = -0.8161f =1, f₃ = -4.516, f₄ = -1.539 ΣP = 0.015, l = 0.212In the above-mentionedrespective embodiments, reference symbols r₁ through r₁₀ respectivelyrepresent radii of curvature of respective lens surfaces, referencesymbols d₁ through d₉ respectively represent thicknesses of respectivelenses and air spaces between respective lenses, reference symbols n₁through n₅ respectively represent refractive indices of respectivelenses, reference symbols ν₁ through ν₅ respectively represent Abbe'snumbers of respective lenses, reference symbol f represents the focallength of the lens system as a whole, reference symbol f₃ represents thefocal length of the third lens, reference symbol f₄ represents the focallength of the fourth lens, reference symbol l represents the workingdistance of the lens system and reference symbol ΣP represents Petzval'ssum.